Make the file_test binary work without custom environment variables. (#5442)

Instead of crashing when run outside of `bazel`, make the toolchain's
`file_test` binary work properly when no test-specific environment
variables are set. This makes it a lot easier to run `file_test` under a
debugger.

There are two main changes here:

- Don't crash if `$TEST_TMPDIR` is unset. Instead, fall back to LLVM's
temporary directory (typically `$TMPDIR`). We already did this in some
places in tests. We now do it in more places.
- Don't fall back to a target label of `<target>` in the reproduction
commands if `$TEST_TARGET` is unset, because this causes all the tests
to fail because their output doesn't match the expected output due to a
differing bazel run command. Instead explicitly specify the target from
the `FileTestBase`-derived class.

Infrastructure for this has been added generally, but only rolled out to
the toolchain `file_test` binary for now.

---------

Co-authored-by: Jon Ross-Perkins <jperkins@google.com>

toolchain/driver/driver_test.cpp

@@ -38,9 +38,7 @@ class DriverTest : public testing::Test {
             InstallPaths::MakeForBazelRunfiles(Testing::GetExePath())),
         driver_(fs_, &installation_, /*input_stream=*/nullptr,
                 &test_output_stream_, &test_error_stream_) {
-    char* tmpdir_env = getenv("TEST_TMPDIR");
-    CARBON_CHECK(tmpdir_env != nullptr);
-    test_tmpdir_ = tmpdir_env;
+    test_tmpdir_ = Testing::GetTempDirectory();
   }
 
   auto MakeTestFile(llvm::StringRef text,